1. Field of the Invention
The present invention relates to waveguide matching networks for extracting electromagnetic energy from a microwave amplification device, and more particularly, to a resonant cavity in series with the output waveguide for enhancing efficiency and bandwidth in klystrons.
2. Description of Related Art
Linear beam tubes such as klystrons and travelling wave tubes are used in sophisticated communication and radar systems which require amplification of an RF or microwave electromagnetic signal. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section, and an output section. An electron beam is sent through the klystron and the electrons are velocity modulated. Those electrons that have had their velocity increased gradually overtake the slower electrons, resulting in electron bunching. The buncher section amplifies the velocity modulation of the electron beam. The traveling electron bunches represent an RF current in the electron beam. The RF current induces electromagnetic energy into the output section of the klystron as the bunched beam passes through the output cavity, and the electromagnetic energy is extracted from the klystron at the output section. An output waveguide channels the electromagnetic energy to an output device, such as an antenna.
The power produced by a klystron is a function of the level of resistance that is generated across the output gap. The integral of the resistance over angular frequency cannot exceed .pi./2C, where C is the input capacity. In the case of a klystron, C is primarily the capacitance of the output gap. If the current produced by the electron beam were independent of gap voltage, then the resistance bandwidth product would approach a theoretical limit defined by Bode's theorem, that is, the integral of Rd.omega. cannot exceed .pi./2C, where R is the resistance and d.omega. is the bandwidth. A detailed description of Bode's theorem is described in his book "Network Analysis and Feedback Amplifier Design," Van Nostrand Company, Inc. 1945 at page 282. In fact, however, the driving current is reduced substantially as the RF voltage developed at the gap begins to exceed the beam voltage. This effect is most pronounced at the band edges where the input impedance of the network has a substantial reactive component. To improve the response, an additional cavity can be coupled to the output. This tends to "square up" the resistance versus frequency characteristic of the gap while minimizing the reactance at the center of the pass-band. Determining the effects of load impedance on output power can lead to further enhancements if an output circuit can be synthesized with the proper voltage standing wave ratio ("VSWR") and phase to maximize power as a function of frequency.
The bandwidth of a klystron can be increased over that produced by a single gap by utilizing output gaps in several cavities coupled together. Energy is extracted from the electron beam in N gaps where 1/N times the total impedance appears at the first gap, and the sum of the voltages at the first gap and succeeding gaps is made roughly equal to the beam voltage by suitable impedance tapering. Bode's theorem again defines the maximal attainable power-bandwidth product, but because the resistive component at the input to the network is lower, one can achieve approximately N times the bandwidth of the single cavity output given similar gap dimensions. As before, based on experimental measurements, power improvements can be realized by the addition of an output network designed to present the optimum phase and VSWR to an output circuit consisting of multiple coupled cavities, each with gaps driven by the electron beam. FIGS. 1 and 2 show the equivalent circuit models for both single and multiple cavity output sections coupled to a terminated waveguide.
Creating a load network that reflects the optimal VSWR and phase found to enhance power across the band is generally a formidable task. In the prior art, most such networks, i.e., waveguide matching networks for broadband klystrons, have utilized shunt susceptances at various distances from the final cavity iris. For example, objects extending part way across the narrow dimension of a TE.sub.10 waveguide produce shunt capacitive susceptances, and objects running completely across the narrow dimension of a TE.sub.10 waveguide produce inductive susceptances. However, there are significant drawbacks to utilizing shunt susceptances for impedance matching. For some devices, the ideal transformer ratio should be higher at the band edges to compensate for the increase in the reactive component of the impedance that occurs away from the center of the pass-band. Furthermore, the position of the shunt element combined with the dispersive characteristic of a waveguide creates a situation where the phase of the impedance generated at the output gap is only optimal over a narrow frequency range. As a result, any performance improvement over one portion of the band can be offset by a corresponding degradation elsewhere (i.e., where the reflected impedance is out of phase). In many cases, it is simply not possible to build a transformer consisting of shunt susceptive discontinuities which optimize the output power of either single or multiple gap klystrons over the desired band of operation.
Accordingly, it would be desirable to provide a system that optimizes the output power of a klystron over the desired band of operation. Such a system would be frequency sensitive and could localize over a frequency range the magnitude of the reflection generated, where the magnitude of the reflection generated positively effects output power. Such a system also would allow for an increase in power over a certain frequency range, and also, because of the decrease in the magnitude of the mismatch outside of this frequency range, reduce negative effects on power caused by out of phase reflections. The system would thus produce higher operating power at designated frequencies and simultaneously increase the bandwidth of the klystron.